Field of the Invention
The present invention relates to wireless communication and, more particularly, to a method and apparatus for performing hybrid automatic repeat request (HARQ) in a carrier aggregation system.
Related Art
Carrier aggregation systems are getting attention these days. A carrier aggregation system refers to a broadband wireless communication system comprising more than one component carrier (CC) which provides bandwidth smaller than the target bandwidth of the broadband wireless communication system. In a carrier aggregation system, a term of a serving cell may be used instead of the term of a component carrier. A serving cell associated with the carrier aggregation system may consist of a pair of two component carriers such as a downlink component carrier (DL CC) and an uplink component carrier (UL CC) or consist only of DL CCs. A carrier aggregation system is such a kind of system where a plurality of serving cells is associated with one user equipment.
Conventional carrier aggregation systems allowed aggregating only those component carriers operating in the same scheme. In other words, the a priori art allowed aggregating only the component carriers operating based on the frequency division duplex (FDD) scheme or the time division duplex (TDD) scheme. In particular, in the case of TDD, it is assumed that component carriers to be aggregated are set up with the same uplink-downlink configuration (UL-DL configuration). UL-DL configuration is intended to inform in which subframe each individual subframe within a frame consisting of a plurality of subframes is used between an uplink (UL) subframe and a downlink (DL) subframe.
However, there are chances that future wireless communication systems may not be constrained to aggregate only those component carriers employing the same scheme. For example, a component carrier (cell) operating based on the FDD scheme or a component carrier (cell) consisting only of uplink subframes may be aggregated into the component carrier (cell) operating based on the TDD scheme. Or a component carrier (cell) operating based on the TDD scheme may be aggregated into a component carrier (cell) operating based on the FDD scheme.
In a TDD cell using component carriers operating based on the TDD scheme, a timing relationship (it is called GU) between a subframe receiving a uplink grant and a subframe to which a uplink data channel scheduled by the uplink grant and a timing relationship (it is called DC) between a subframe receiving a downlink data channel and a subframe transmitting ACK/NACK (acknowledgement/not-acknowledgement) about the downlink data channel are determined according to uplink-downlink configuration (UL-DL configuration).
In conventional systems, the timing relationships, namely, GU and DC, have been determined according to UL-DL configuration provided through system information; however, in future wireless communication systems, the GU and the DC may be configured differently from the UL-DL configuration given commonly to the corresponding cell by the system information.
Meanwhile, since a DL subframe and a UL subframe may not exist in pairs within a frame operating in a TDD cell, one UL subframe may have to transmit ACK/NACK to a plurality of DL subframes. For error-free transmission of ACK/NACK, the number of DL subframes actually scheduled among the plurality of DL subframes should be known when one UL subframe transmits ACK/NACK. The number of DL subframes actually scheduled is informed through an uplink grant scheduling the one UL subframe. Since DC is larger than or equal to GU according to the UL-DL configuration of a conventional TDD cell, the number of DL subframes actually scheduled can be known through an uplink grant. It is also the case for an FDD cell.
However, in case the GU and the DC in the future wireless communication system are configured differently from the uplink-downlink configuration given by the system information, there are times when GU becomes larger than DC. At this time, if an uplink grant attempts to inform of the number of actually scheduled DL subframes, scheduling of a downlink data channel should be predicted. If the prediction is inaccurate, an error may occur in ACK/NACK transmission.